High density polyethylene (“HDPE”) resins are used in numerous blow molding applications to make products such as household and industrial chemical containers (“HIC”). These resins generally have good rheological properties, but generally poor environmental stress crack resistance (“ESCR”). Accordingly, there is a need to improve the ESCR of certain HDPE, including chromium-catalyzed HDPE. The prior art contains various blends of resins purporting to increase ESCR in polymers, and general methods of selecting the resins which comprise the blends. One such approach is to select resins which have a broad orthogonal composition distribution (“BOCD”). In a BOCD, comonomer is distributed more evenly in the high and low molecular weight fractions of the resin, which is different from the comonomer distribution in resins made from Ziegler-Natta catalysts. For example, such blends of BOCD resins are generally disclosed in U.S. Pat. No. 5,382,630 to Stehling et al., which is fully incorporated herein by reference.
Various publications disclose blending polymers to improve ESCR. For example, WO 2004/016688 to Harris et al. discloses a melt blend of a linear low density polyethylene resin and/or a linear medium density polyethylene resin, and a high density polyethylene resin. The polyethylene composition has a density of about 0.945 to about 0.960 g/cc and a melt flow index of about 0.1 to about 0.4.
U.S. Pub. 2004/0063861 to Lustiger et al. discloses polyethylene compositions having a first polyethylene having a melt index of 0.1 to 3.0 g/10 min, and a density of from 0.905 to 0.938 g/cc; and a second polyethylene having a melt index of 10 to 500 g/10 min and a density of 0.945 to 0.975 g/cc. The composition has a density of from 0.920 to 0.973 g/cc and a melt index of 2 to 200 g/10 min, and the density of the second polyethylene is from 0.037 to 0.062 g/cc greater than the density of the first polyethylene. These compositions purportedly exhibit improved physical properties, such as ESCR, relative to conventional compositions of similar melt index and density.
U.S. Pub. 2003/0088021 to Van Dun et al. discloses blended compositions containing a homopolymer, the use of which allows the incorporation of more comonomer in the additional components of the blend (for the same overall density), resulting in increased tie molecule formation which is said to improve properties such as ESCR, toughness and impact strength.
WO 2003/051937 to Maziers discloses a process for the preparation of polyethylene resins having a multimodal molecular weight distribution that comprises the steps of: (i) providing a first high molecular weight metallocene-produced linear low density polyethylene resin having a density of from 0.920 to 0.940 g/cc, and a high load melt index of from 0.05 to 2 g/10 min; (ii) providing a second high density polyethylene prepared either with a Ziegler-Natta or with a chromium-based catalyst, and polyethylene having a density ranging from 0.950 to 0.970 g/cc, and a high load melt index of from 5 to 100 g/10 min; (iii) physically blending together the first and second polyethylenes to form a polyethylene resin having a semi-high molecular weight, a broad or multimodal molecular weight distribution, a density ranging from 0.948 to 0.958 g/cc and a high load melt index of from 2 to 20 g/10 min. The molecular weight of the metallocene-produced resins is very large and can be typically from 400,000 up to 1,500,000. The high molecular weight metallocene-produced resin will cause the blended composition to have different rheological properties as compared to the base chromium-catalyzed polyethylene. Moreover, the difficulty of dispersing the blended components into each other increases as the molecular weight of each component increases.
In improving ESCR for a blended composition wherein the majority component is a chromium-catalyzed polyethylene, however, the above methods would provide a blended composition having a density much lower than the majority component and/or the rheological properties of the blended composition would be substantially different from that of the majority component. It is important to maintain the density in order to maintain beneficial physical properties such as, for example, stiffness. Accordingly, what is needed is a polymer composition and method of making the same that has improved ESCR over the base chromium-catalyzed polymer without decreasing the density of the blend too far below the density of the base chromium-catalyzed polymer or substantially changing the rheological properties of the base chromium-catalyzed polymer. Additionally, when a metallocene-catalyzed polyethylene is used in the composition, it is desirable that it have a low molecular weight to improve dispersion and to improve the rheological properties of the blend.
Additional references of interest include: “The Search for New-Generation Olefin Polymerization Catalysts; Life Beyond Metallocenes”, Angewandte Chemie Int. Ed., Vol. 38, pp. 428-447 (1999), authored by George J. P. Britovsek, et al.; “Novel olefin polymerization catalysts based on iron and cobalt,” Chem. Comm., pp. 849-850 (1998), authored by George J. P. Britovsek, et al.; “Sterically Demanding Diamide Ligands: Synthesis and Structure of do Zirconium Alkyl Derivatives,” Organometallics, Vol. 14, pp. 5478-5480 (1995), authored by D. H. McConville, et al.; “Heterogeneous Single Site Catalysts For Olefin Polymerization”, Chem. Review, Vol. 100, pp. 1347-76 (2000), authored by G. G. Hlatky; WO 01/98409; WO 03/051937; WO 97/48735; WO 98/46651; WO 96/33227; WO 96/27439; WO 96/23010; WO 97/22639; WO 96/11961; WO 96/11960; WO 96/08520; U.S. Publication Nos. 2003/0088021 and 2004/0063861; WO 2004/016688; EP 0314385; EP 0816384; U.S. Pat. Nos. 3,231,550; 3,242,099; 3,248,179; 3,622,521; 4,429,079; 4,438,238; 4,461,873; 4,543,399; 4,547,551; 4,588,790; 4,613,484; 4,808,561; 5,001,205; 5,017,714; 5,028,670; 5,055,438; 5,064,802; 5,082,902; 5,124,418; 5,153,157; 5,306,775; 5,317,036; 5,319,029; 5,324,800; 5,352,749; 5,371,146; 5,380,803; 5,382,630; 5,382,631; 5,405,922; 5,436,304; 5,453,471; 5,462,999; 5,502,124; 5,504,049; 5,616,661; 5,668,228; 5,712,352; 5,851,945; 5,852,146; 6,090,893; 6,180,721; 6,376,410; 6,380,122; and JP 1997194537.